#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <map>
#include <set>
#include <unordered_map>
#include <unordered_set>
#include <climits>
#include <iomanip>
#include <cmath>
#include <string>
#include <vector>
#define int long long
#define x first 
#define y second 
using namespace std;
const int N = 1e5 + 10;
int a[N];
void solve()
{
    int n,k;
    cin >> n >> k;
    int p = n * (n - 1) / 2;
    cout << p - k << endl;
}
signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr),cout.tie(nullptr);
    int t = 1;
     //int t;
     //cin >> t;
    while(t -- ) solve();
    return 0;
}
/* 长度为n的序列，一共最多有(n - 1) * n / 2个逆序对 */